The present invention relates generally to staked or self-clinching fasteners and methods of manufacture thereof. More particularly, the present invention concerns an improved self-clinching fastener and method for making such a fastener, which fastener enables obtaining a predicted, predetermined locking action with respect to a section of sheet material to which the fastener is to be attached or staked in use.
Self-clinching fasteners, also variously called staked, self-locking and counter-sinking fasteners, are generally known in the art, and are of numerous designs. These fasteners employ various clinching or staking methods for connection to sections of sheet material or other similar workpieces. Many such prior art fasteners are of the type which are assembled in a preformed hole in a sheet material, whereupon the head portion of the fastener is seated and embedded in a facing surface of the sheet material to cause displacement or cold flowing of a portion of the sheet material radially inwardly of the preformed aperture therein. This displacement is such as to cause the material to flow into intimate contact with facing surfaces of the fastener including a locking groove formed therein, thus providing means for maintaining the fastener in relatively rigid engagement with the sheet material.
The art has provided a number of structures for variously forming undulations, locking grooves or other surface features on the fasteners for engagement with the flowing material of the sheet material during seating and embedding of the fastener to achieve the desired mechanical interlock or engagement between the fastener and the sheet material.
One particularly useful and successful form of such a prior art fastener is shown in U.S. Pat. No. 3,967,669. As explained in this patent, the fastener thereof employs an extruding section of larger diameter than the preformed aperture in the sheet material, which is formed about the shank of the fastener axially spaced from the underside of the head. It is the function of this extruding section or "sizing ring" as it is sometimes called, to essentially resize the aperture to a predetermined larger diameter such as to assure that a sufficient amount of material of the sheet will be deformed into close engagement with the locking surfaces and/or locking features of the fastener upon the seating and embedding of the fastener in the sheet material.
As further discussed in the above-referenced prior U.S. patent, the degree or strength of the mechanical interlock or connection between the fastener and sheet is dependent upon the shear area achieved by the mechanical interlock. This shear area is generally defined as the cross-sectional area of the material displaced into the locking groove or other locking feature of the fastener, taken in the direction of shearing, that is axially. In this regard, the fastener of the above-referenced '669 patent includes a locking groove comprising a generally annular groove which is defined intermediate the underside of the head portion of the fastener and the enlarged diameter extruding section or sizing ring. Accordingly, it has heretofore been deemed desirable to make the width of the locking groove as great as possible in order to maximize this axial shear area. However, with thin sheet materials, it is not possible to employ a relatively wide groove, since insufficient sheet material will be present to deform into close contact with the outer surface of the groove throughout a relatively wide groove.
In the above-referenced '669 patent, the extruding section or sizing ring was designed to overcome the above-described problem with relatively thin sheet materials, by permitting reworking of the sheet material aperture to displace an increased amount of material into the locking groove. This enabled employment of a fastener with a relatively wide locking groove with relatively thin sheet materials. That is, a relatively small aperture could be initially formed in the sheet material, whereupon the extruding section or sizing ring would greatly enlarge and reform the aperture resulting in a significant quantity of additional material from about the former smaller aperture being displaced and available to be formed into engagement with the locking groove.
The fastener of the '669 patent also employs a locking feature comprising a first series of arcuately curved surfaces and a second series of arcuately curved surfaces which are curved oppositely of the first series and alternate therewith while merging smoothly therewith. That is to say, the surfaces of one of the series of surfaces are generally concave whereas the surfaces of the second series are convex viewed in the radial direction relative to the head and shank of the fastener. These surfaces project axially from the underside of the head and radially from the shank. Preferably, the first and second series of arcuately curved surfaces are each six in number. Preferably, the particular design or configuration of these arcuately curved surfaces is that disclosed in U.S. Pat. No. 3,584,667.
The present invention improves yet further upon this prior art arrangement by providing an annular undercut groove in a staked locking fastener, said groove radially surrounding the shank portion of the fastener where it extends from the head. The depth of the undercut groove is preferably at least one-half of the axial height of the locking projections, which may take various forms. As will be more fully described hereinbelow, this undercut groove will become filled with the deformed material of the sheet upon clinching or embedding of the fastener into the sheet material. In the present invention, a sizing ring is of a greater outer diameter than the inner diameter of this annular undercut groove and is axially spaced from the groove. As such, additional axial shear area is defined, that is, generally in the axial direction of the depth of the groove thus even further increasing the shear area, and consequently, even further strengthening the mechanical connection between the fastener and the sheet material.
As an additional matter, we have found that the formation of the annular undercut groove further facilitates and enhances the formation of the above-discussed type of locking feature, in a fastener employing a locking feature of the type. That is, we have found that with the above-discussed undercut annular groove, the arcuately curved surfaces of the locking feature or projections can be more clearly and sharply defined particularly in their radially outermost axially extending faces. This formation of the axial or vertical side walls of the arcuately curved surfaces or "displacement lobes" as they are sometimes termed, provides increased resistance to either clockwise or counterclockwise motion of the fastener relative to the sheet material and hence an increase in unsupported torque resistance.